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Typical KT88 impedance?

3/24/2004 9:37 PM
Heiko Frost Typical KT88 impedance?
Hey there,  
can anyone tell me what would be a typical output transformer impedance for a fixed bias 4 x KT88 amp?  
3/24/2004 9:51 PM
Ray Ivers

The GEC KT88 amp design book shows an AB1 UL 100W circuit for two KT88's, using 4K for 460V B+ and 4.5K for 560V, and they also have an 8 x KT88 AB1 circuit using a 1K primary Z @ 560V B+. From this, I would say that any good 100W OT around 2K primary Z should work fine with 4 x KT88's at 460 to 600V B+.  
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3/24/2004 10:09 PM
Heiko Frost
Thanks Ray. I appreciate the info.  
3/25/2004 6:13 PM

Hey Ray, a few folks in here advised me against using my OEI 100W tranny w/ 4xKT88's running at 560Vdc because they said I would generate 200W. Forgive me for pestering, but I put off a build for the expectation of needing a 200W OPT. So would you say that I wouldn't blow the tranny w/ that arrangement (provided I didn't wire it funky)?  
3/26/2004 12:08 AM
M. Taylor
Glenn, with 4 KT88's in fixed bias with 560Vdc on the plates I image you'll probably make a heck of a lot more power than 100W. Though I'm sure there's a fair safety margin designed into OEI's transformer I think you might smoke it... What about using a pair with the impedance selector set to expect half of your cabinet's Z (for a 3500 ohm primary)?  
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3/26/2004 1:06 AM
Ray Ivers

Why not shoot Joe at OEI an E-mail and ask him directly? Or maybe he'll read this thread and join in before too long...  
I should have just stuck to answering Heiko's primary-Z question since I don't know what power supply he'll be using or if he intends to use the amp for bass guitar, and it's possible (but not likely IMO) that a super-beefy PS unlike that in most guitar amps coupled with an OT on the marginal side of "good" might have had some problems if used long-term for bass guitar. So, I'll revise my post as follows: " in a 4 x KT88 output stage running from 450V to 600V B+ with sufficient full-load power supply current capability to support 200 watts of audio output power, a Hammond model 1650T UL output transformer should work fine for guitar-amplifier (not bass guitar or hi-fi) purposes, and a Hammond 1650W UL output transformer should work well for guitar, bass guitar, and hi-fi uses".  
3/26/2004 1:51 AM

Thanks for the reply! And I wouldn't try to hold you 'responsible' for anything I do w/ an amp because that responsibility rests upon me and me alone (if I don't have a good idea about what I'm attempting, then I have no business doing it).  
I guess my confusion lies in the fact that 6550's and KT88's are always linked together as the 'same' type of tube, and the EL34's and 6550's were exchanged freely in those Marshall amps (and given the high voltages for some of those amps) I couldn't really see the harm. But that's just what fools tell themselves, isn't it? LOL! Thanks again, I shall pester Joe some more!  
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